The invention relates generally to energy-based systems, and in particular to a method of computing the AC impedance of these systems.
It is known in many applications, including self-propelled vehicle applications as seen by reference to U.S. Pat. No. 6,394,208 entitled xe2x80x9cSTARTER/ALTERNATOR CONTROL STRATEGY TO ENHANCE DRIVEABILITY OF A LOW STORAGE REQUIREMENT HYBRID ELECTRIC VEHICLExe2x80x9d issued to Hampo et al., to employ a dynamoelectric machine in a first mode as a motor in order to provide propulsion torque. In such applications, it is also known to reconfigure the dynamoelectric machine in a second mode as a generator, in order to capture and convert some of the potential or kinetic energy associated with the application into output electrical power, a process known as regeneration (xe2x80x9cregenerative energyxe2x80x9d). Moreover, in such applications, it is also known to provide an energy system, such as a battery, to power the dynamoelectric machine when operated as a motor, and to receive the regenerative energy when the dynamoelectric machine is operated as a generator. In the latter case, the regenerative energy is generally operative to increase the state of charge of the battery, until such battery is xe2x80x9cfullyxe2x80x9d charged. Battery technologies typically used in such applications include nickel metal hydride (NiMH), lead acid (PbA) and nickel cadmium (NiCd) technologies, although energy systems employing lithium chemistry technologies, while not as prevalent as other battery technologies, are also used in practice.
One aspect of the above systems that involves tradeoffs or compromises pertains to optimizing the performance of the energy system based on the condition of the energy system itself. One method of evaluating the condition of the energy system is by computing and analyzing the AC impedance of the energy system. As an energy system discharges its energy, its AC impedance rises meaning that the system is more resistant to giving up its remaining charge. The ability to compute this AC impedance can be beneficial for a number of reasons. By taking a xe2x80x9csnapshotxe2x80x9d of the impedance of the energy system immediately following the charging of the system, and then continuously computing the impedance as the system is used, the system operator can evaluate the condition of the energy system by comparing the two values, and therefore, observe whether the energy system is xe2x80x9cfreshxe2x80x9d, or whether it is aging (e.g., losing energy storage capacity). Therefore, computing AC impedance is useful for diagnostics, energy system control, and in determining the condition of the energy system. It can allow the energy system controller to more accurately predict energy system behavior, which allows for more precise system control, enhanced performance, and more optimized charge and discharge profiles. It also allows for the system controller to compensate for energy system aging.
Current methods of computing the impedance of an energy system include using DC based measurements. These methods compute impedance by using two data points taken at very different currents, and then computing the impedance by calculating the difference of the voltage divided by the difference in the current. Other current methods include exciting the energy system with a low-level AC waveform, and then measuring the magnitude and phase of the resulting excitation current to compute the AC impedance. However, this method requires a separate circuit for generating the excitation waveform.
These existing methods, while adequate, do not allow for the most useful method of optimizing the AC impedance computation. Existing methods, as set forth above, do not provide for the xe2x80x9creal timexe2x80x9d computation of AC impedance without the addition of external hardware or specialized processes as the system is live in operation.
There is, therefore, a need for a process that allows the energy system to self-compute its AC impedance while it is live in operation that will minimize or eliminate one or more of the above-identified problems.
An object of the present invention is to solve one or more of the problems as set forth above. One advantage of the present invention is that the energy system, which may be formed of a fuel cell or any bi-directional energy storage devices such as lithium chemistry technologies, is able to continuously compute its AC impedance in xe2x80x9creal time,xe2x80x9d thereby allowing it to constantly monitor and optimize its performance either while the energy system is xe2x80x9clivexe2x80x9d in operation, or while the system is xe2x80x9coff-line.xe2x80x9d The present invention allows the energy system to perform internal complex impedance measurements to thereby determine the AC impedance of the energy system. This AC impedance computation then allows for improved diagnostics and control of the energy system.
Another advantage is that the measurements made in accordance with the present invention are done so in xe2x80x9creal-timexe2x80x9d as the system is live in operation without requiring any additional testing or monitoring circuitry, or any user interaction.
These and other features, objects, and advantages are realized by the present invention, which includes a method of computing the AC impedance of an energy system. The method includes measuring the electrical characteristics of power (i.e., current and voltage) drawn by a given application coupled to the energy system, and storing the measurements in the memory of the energy system. The method further includes processing the measurements to develop an impedance parameter value that is indicative of the energy system""s AC impedance, and then storing this parameter value to a memory buffer.
An energy system according to the invention is also presented.